1. Field of Invention
This invention relates generally to optical micrometer systems for precisely measuring minute distances between spaced points on an object being tested, and more particularly to a system of this type which produces an illuminated aerial image of the object and optically superimposes thereon an illuminated image of a pointer whose position is shiftable by an operator to determine the distance travelled by the pointer image in going from one point to another on the object image.
2. Prior Art
Though the invention is applicable to the measurement of minute distances between any two points or lines on an object such as a biological specimen or a precision-machined part, it has particular value in conjunction with microelectronic or integrated circuit devices in which the passive and active electrical components of a circuit such as solid-state diodes or transistors are created on a silicon substrate, the components being interconnected by a pattern of conductive lines formed on the substrate.
In test procedures related to the production of such microelectronic devices, one must measure the width of the various conductive lines which are included in the line pattern as well as the spacing between lines. In this rapidly evolving field, in order to reduce the cost and dimensions of such devices, the direction is toward a greater density of components on a given substrate, this being accompanied by a compression of the conductive line pattern. As a consequence, the conductive line formations are thinner, with closer spacings therebetween.
Since it is vital that line widths and spacings in integrated circuits conform with design specifications, the formation of finer lines and closer line spacings has further narrowed the acceptable margin of error in measurement techniques for determining whether the line pattern is in compliance with the specifications therefor.
It is known to use optical micrometers to carry out measurements on integrated circuits. Two types of optical micrometers are currently in widespread use. One is the so-called filar micrometer, such as the Digital Filar Micrometer Eyepiece manufactured by E. Leitz, Inc. of Rockleigh, N.J., and the other being the image-shearing micrometer, such as the Image-Shearing Measuring and Direct Dimensional Read-Out System manufactured by Vickers Instrument, Inc. of Malden, Mass.
In a filar micrometer, a cross hair of filar is physically placed at the real image plane of a compound optical microscope and both are imaged on the retina of the eye. The motion-producing mechanism for the filar is operatively coupled to a digital electronic micrometer for measurement of line width or line spacing on the object image.
Because the operator, via his eye, must determine the placement of the filar, this requirement often gives rise to erroneous readings. In practice this type of measurement is subject to a fairly high order of human subjectivity. No two operators possess the same degree of visual acuity, and operator fatigue and the optophysiological requirements for repeatability are such as to render the filar micrometer a relatively imprecise measuring instrument.
A further problem with a filar micrometer is that there is generally insufficient access to the real image plane for the filar and the associated micrometer mechanism. The physical presence of the filar and micrometer mechanism in the image plane renders standard binocular vision inconvenient.
In the image-shearing optical micrometer, a beam-splitter acts to divide the imaging light beam into two parts and to create two identical images of the object under test. These images are shifted or sheared with respect to each other by an amount controlled by a micrometer. The degree of shear is directly proportional to rotation of the micrometer drum and can be read from a digital read-out associated therewith.
When the shear is zero, the illuminated double images of the object are exactly superimposed and are seen through the eyepiece as a single, relatively dark image against a bright field. If the shear is such that the two images overlap, then the amount of shear is less than the object dimension, this being indicated by the fact that the overlapping zone is as dark as superimposed images, whereas the non-overlapping areas of the two images are less dark. When, however, the two images just touch each other, each image is less dark than in the case of superimposed images, and the amount of shear is exactly equal to the object dimension. Thus when the object being tested is a photo-mask of a conductive line pattern of an integrated circuit, the condutive lines in one image will be contiguous with the corresponding conductive lines in the other image only when the amount of shear is exactly equal to the line thickness.
Since the precision of the image-shearing measurement technique depends on finding the point at which the images just touch--and this is indicated to the operator in terms of light intensity--in practice the subjectivity of the typical operator makes it difficult to discern this exact point. Also, due to the doubling of every image point, this leads to confusion and fatigue.
The following prior art patents are of interest in connection with the present invention: Boughton, U.S. Pat. Nos. 3,582,178; Vanden Brack et al., 4,099,881; McGivern, 3,398,631 and Fassin, 1,974,606.